The present specification relates to displays. More particularly, the present specification relates to head up displays (HUDs).
Conventional HUDs utilize an image source and a combiner to provide an image (e.g., symbology, enhanced vision, etc.) to a user. The user (e.g., a pilot) can view the image on the combiner as well as an environmental scene through the combiner. Substrate guided HUDs have been proposed which use waveguide combiners to preserve eye box size while reducing lens size. U.S. Pat. No. 4,309,070 issued to St. Leger Searle and U.S. Pat. No. 4,711,512 issued to Upatnieks disclose waveguide HUDs.
HUDs can be susceptible to a loss of contrast ratio in certain environmental conditions. The loss of contrast ratio can be due to bright light in the environment often associated with direct lunar light, direct solar light, lunar reflection or solar reflection. The bright light can strike the combiner or other optical components and dominate the background and/or image or portions thereof. In substrate guided HUDs, the loss of contrast ratio can also be due to interference when the diffractive gratings in substrate waveguides are exposed to bright light.
The susceptibility to loss of contrast ratio is increased in glass cockpit environments, such as, those associated with tactical aircraft. For example, the bright light (e.g., from the sun, bright clouds, the moon and reflections) can directly strike the combiner or other optical components through the windshield, glass canopy, windows, etc. and overcome or saturate the background and/or image or portions thereof. The saturation can occur at specific points or areas on the combiner and does not necessarily affect the entire display. Conventional uses of tinted lens and visors can reduce background saturation, but do not adequately solve lower contrast ratio issues.
Accordingly, there is a need for a HUD that is less susceptible to lower contrast ratios caused by non-uniform bright light from the environment. Further still, there is a need for a HUD having a waveguide combiner that is less susceptible to solar and lunar specular reflection. There is also a need for a system and method of eliminating or mitigating image quality degradation due to bright light in the environment. Further still, there is a need for a combiner which can mitigate the effects of refractive saturation on image quality of a waveguide display. Further still, there is a need for an inexpensive, lightweight solution for preventing or reducing solar or lunar light saturation of the background scene.